
$C_{\text {net }}=\frac{k+1}{2} \frac{A \varepsilon_0}{d}$
$C_{\text {net }}=\frac{A \varepsilon_0}{d-t\left(1-\frac{1}{2}\right)}=\frac{A \varepsilon_0}{d-\frac{t}{2}}$
$C_{\text {net }}=\frac{3 A \varepsilon_0}{2 d}$
$\frac{3 A \varepsilon_0}{2 d}=\frac{A \varepsilon_0}{d-\frac{t}{2}}$
$1.$ A sphere $2.$ Cylindrical
$3.$ Pear $4.$ Lightning conductor
are mounted on insulating stands and charged. The one which is best suited to retain the charges for a longer time is
$\varepsilon(x)=\varepsilon_{0}+k x, \text { for }\left(0\,<\,x \leq \frac{d}{2}\right)$
$\varepsilon(x)=\varepsilon_{0}+k(d-x)$, for $\left(\frac{d}{2} \leq x \leq d\right)$

